The development of novel catalytic systems exhibiting unique reactivity and high enantioselectivity requires the synthesis of chiral ligands for transition metals. Generally, some of the most successful chiral ligands have been chelating phosphines possessing a C.sub.2 symmetry axis.
Many of the chiral phosphines known in the art have at least two aryl substituents on the phosphorous, rendering that center relatively electron-poor. The mechanism of asymmetric induction using these phosphines has been linked to the proper conformational relationship between the phenyl groups on the phosphorous centers.
More recently, chiral phosphines having relatively electron-rich phosphorus centers have been reported. Brunner et al., Journal of Organometallic Chemistry, Vol. 328, PP 71-80 (1987) teach 3,4-disubstituted phospholanes derived from tartaric acid having chloro, methoxy, or dimethylamino substituents. These were complexed with manganese and rhodium and used as catalysts in the hydrogenation of alpha-N-acetamidocinnamic acid. Relatively low optical yields of (S)-N-acetylphenylalanine of from 6.6% enantiomeric excess to 16.8% enantiomeric excess were obtained.
S. R. Wilson and A. Pasternak, Synlett, April, 1990, pp 199-200 describe the preparation of (2R,5R)-1-phenyl-2,5-dimethylphospholane and its use in an enantioselective Staudinger reaction (reduction of azides with phosphines). Here the chiral (2R,5R)-1-phenyl-2,5-dimethylphospholane is used as a stoichiometric reactant, not as a catalyst.
M. J. Burk et al, Organometallics, Vol 9, pp 2653-2655 (1990) describe a series of mono and bidentate 2,5-disubstituted phospholanes and demonstrate their use as ligands in asymmetric catalysis. Rhodium complexes bearing the disclosed phosphine ligands were prepared and tested as catalyst precursors for the enantioselective hydrogenation of unsaturated substrates. The phosphorous atoms in the disclosed bis(phospholanes) are linked by two- or three- carbon methylene bridges. There is no indication nor suggestion that these bridges linking the phosphorus atoms might contain heteroatoms. There is no indication nor suggestion that a bis(phospholane) where the bridge linking the phosphorus atoms contains heteroatoms would be able to complex transition metals in a tridentate fashion.
M. J. Burk et al, Angewandte Chemie, International Edition in English, Vol 29, pp 1462-1464 (1990) disclose tris(phospholane) tridentate ligands with C.sub.3 symmetry. There is no indication nor suggestion that a bis(phospholane) where the bridge linking the phosphorus atoms contains heteroatoms would be able to complex transition metals in a tridentate fashion.
U.S. Pat. No. 5,008,457 issued Apr. 16, 1991, discloses mono, bidentate and tridentate phospholanes useful as transition metal ligands in assymetric catalysis and processes for their preparation as in the above two Burk et al. references.
A continuing need exists for transition metal complexes providing high levels of stereochemical control and asymmetric induction in stoichiometric and catalytic transformations. There are no known examples of chiral tridentate phospholane ligands which can coordinate in a meridinal fashion on an octahedral metal or which coordinate in square-planar environments. The tris(phospholanes) of Burk et al., supra, are tridentate ligands, but cannot coordinate in this fashion.
A need also exists for efficient synthetic routes for the preparation of chiral ligands having a high degree of enantiomeric purity for transition metal catalysts.
It is therefore an object of the present invention to provide novel tridentate bis(phospholane) compounds as ligands for transition metals.
It is a further object of the present invention to provide tridentate bis(phospholanes) which coordinate in a meridinal fashion on an octahedral metal or which coordinate in square-planar environments.
It is a further object of the present invention to provide transition metal catalysts which provide high levels of stereochemical control in reactions.
It is a further object of the present invention to provide transition metal catalysts which result in high levels of asymmetric induction in hydrogenation reactions.
It is a further object of the present invention to provide efficient synthetic routes for the preparation of these bis(phospholane) compounds.